KR20130100890A - Thermally compensating lens for high power lasers - Google Patents

Abstract

A method of thermally compensating lenses in an optical system for high power lasers includes providing a fused silica lens to collimate a high power laser beam and positioning the lens in a collimated relationship with the laser beam. The focus lens assembly is provided to focus the collimated laser beam and is positioned in focusing relationship with the collimated laser beam. At least one lens with negative dn / dT is included as part of the collimating lens assembly and part of the focus lens assembly to counteract the thermally induced change in refractive index of the fused silica lens. A lens with negative dn / dT is selected from the group of glasses with negative dn / dT. The power of the lenses is balanced with a negative dn / dT lens that is offset so that the optical system maintains focus over a wide temperature range.

Description

Thermal Compensation Lens for High Power Lasers {THERMALLY COMPENSATING LENS FOR HIGH POWER LASERS}

Cross reference of related applications

This application claims priority to currently pending US Patent Provisional Application No. 12 / 756,642, filed April 8, 2010, entitled “Thermal Compensation Lens for High Power Lasers,” which is incorporated herein by reference. do.

The technical field of the present invention

The present invention relates generally to laser technology. More specifically, the present invention relates to a lens for high power lasers.

One or more high quality fused silica lenses are used to focus high power fiber lasers in excess of 1 kilowatt (1 microwatt). More specifically, at least one high quality fused silica lens is used to collimate the laser light emitted from the fiber laser, wherein the fiber is from 50 microns to 300 microns in diameter. After the light is collimated, the light is directed to a focus lens assembly consisting of one or more high quality fused silica lenses that focus the light on the surface to be cut, drilled, scribed, marked or welded.

The fused silica lens material is highly permeable and some radiation is absorbed or scattered within the lens causing the lens to heat up. All optical glass materials have specific thermal properties that change the focus characteristics of the lens when they are heated. In particular, the change value dn / dT of the refractive index n as a function of the coefficient of thermal expansion α and the temperature changes the power of the lens. The power of the lens is affected by the two properties, referred to as the thermal power of the lens:

Therefore, the power of the lens is changed as a function of temperature by the following equation:

Φ is the power of the lens. Thereafter, the change in power is the product of the original power of the lens and the thermal power Ψ _P of the lens.

Fused silica has a very low coefficient of thermal expansion, very high transmission and low scattering qualities over wavelengths from ultraviolet to near infrared in the electromagnetic spectrum. This is the most cost-effective working glass at present. However, as indicated by the preceding equations, with all other optical glasses, it is sensitive to focal power changes as the temperature of the glass increases. The problem of focal power changes due to heat has become a problem for conventional fiber lasers with average powers in excess of 20 kW.

The coefficient of thermal expansion (CTE) for fused silica is about 0.5 × 10 ⁻⁶ / ° K and has dn / dT up to 10 × 10 ⁻⁶ / ° K. Fused silica lenses with a nominal focal length of 200 mm have a change of focus of 350 microns or more for a 100 ° C. temperature increase. This is not a tremendous amount for this long focal length lens, but when used as a collimator for the fiber, it has a significant effect on how light is collimated from the fiber.

Paper written by Abt et al, Focusing High - Power , Single Mode Laser Beams , Photonics Spectra Magazine, May 2008 discuss this problem and show focus shifts between 1 and 2 mm over a 100 watt to 900 watt power range of laser power using various fused silica lenses and gradium index glasses. . Steele et at is a paper published by the Department of Energy, Spot Size and Effective Focal Length Measurements for a fast Axial Flow In the CO2 Laser describes a similar action and CO2 laser. Thermal Lenses in Window Materials by Klein for High - Energy Laser Windows : how thermal Lensing and Thermal Stress Control Performance , SPIE Proceedings Vol. 6666, 66660Z1 (2007).

Conventional methods for handling thermal lenses in optical systems have been to refocus the collimating optics and focusing optics after thermally stabilizing the system for 3 or 4 minutes. This is not very desirable and incurs costly delays in production environments.

US Pat. No. 5,128,953 discloses a method that assists in cooling the lens by placing a small gap between the focal lenses and the debris shield. This method is useful for low power lasers. The collimation problems of high power fiber lasers are not solved unless the high power laser adds additional optics that should be avoided. The prior art requires additional windows, i.e. additional optics, without improving optical performance and without providing a means for providing cooling gas when a multi-element lens is required.

European patent application EP 1 791 229 A1 discloses a method of reducing thermal lenses using stress birefringence and radial polarization in ZnSe. This approach has very limited utility and is not feasible for non-polarized high power fiber lasers.

Non-athermalization is commonly applied to mid-infrared optical systems but not particularly to high power lasers. Such systems are configured to compensate for thermal changes over a broad spectrum in the infrared. Non-thermal activation is modern by Smith Optical Engineering , McGraw Hill 2000 and Practical Optical System Layout and Use of Stock Optical Lenses , McGraw Hill, 1997, and by Fishcer et al. System Design , McGraw Hill, 2008. This document teaches achromatizing and non-thermal activation by solving three simultaneous equations:

Φ is the power of the lens system, Φ _a and Φ _b are the powers of the individual lens elements; φ _a and φ _b are the color powers of each element, and Ψ _a and Ψ _b are the thermal powers of the lenses. The color power of the lens is the inverse of the abbe value v.

With respect to the prior art considered entirely at the time the invention is formed, it will not be apparent to those skilled in the art how the limitations of the prior art can be overcome.

Long, but not so far, demands that have not been realized for the collimation means for high power lasers are now met by the novel, useful and unclear invention.

The configuration of the present invention includes a thermal compensation lens assembly consisting of two or more optical elements, wherein the first element compensates for the thermal power of the second element in order not to change the overall power of the system. In the system of the present invention, further optics provide means for improving the optical performance of the lens system and for providing a cooling gas.

The present invention takes advantage of the thermal advantages of fused silica and improves the prior art by offsetting the change in refractive index according to the second material with negative dn / dT. Most glasses experience a positive change in refractive index as the material's temperature increases, but some glasses, such as CaF2, BaF2, LiF2, NaCl, KCl, have a negative dn / dT. Other glasses have a negative dn / dT, and the invention is not limited to the glasses listed herein for illustration. By balancing the power of the lenses in a system with offset dn / dT, the optical system maintains its focus over a wide temperature range.

As mentioned above, the power of the two lens system is provided as follows:

Φ _a is the power of the first element and Φ _b is the power of the second element. When the lens system changes temperature, its power is changed as follows:

The equation is the sum of the power of each element multiplied by its thermal power Ψ. In order to achieve zero shift of focus, the powers of the two lenses must be balanced as follows:

The laser is a monochromatic light source and therefore is independent of the color power of the lens, all of which are required to non-thermally activate the laser focus lens. An ideal situation is where the expansion coefficient α of each glass is the same and the dn / dT of the first material is the exact negative value of the second material. This anomaly is not feasible, but the offsets in α and dn / dT are compensated by changing the thickness and curvature of the elements to achieve more accurate absolute values of each ΦΦ.

The main object of the present invention is to maintain a constant focal position within the Rayleigh distance (focal depth) of the system when the temperature changes between ambient temperature and thermally stabilized temperature.

Another object is to provide thermal management for the optical elements to dissipate the heat absorbed when the laser power is increased.

These other important objects, advantages and features of the present invention will become apparent as the description proceeds.

Thus, the present invention includes the features of the constructions, combinations of elements and arrangement of parts to be illustrated in the following description, the scope of the invention will appear in the claims.

For a more complete understanding of the features and objects of the present invention, reference will be made to the following detailed description in conjunction with the accompanying drawings:
1 shows a high power fiber laser light of the prior art, radiated from the output of a fiber and focused again on one surface.
2 shows the lenses of FIG. 1 with a multi-column configuration.
3 is an enlarged view of the focus of FIG. 2.
4 shows a conventional fiber collimator for a high power fiber laser.
FIG. 5 is a spot diagram of the collimation lens doublet shown in FIG. 4.
6 shows that the present invention is configured as a fiber collimator for a high power fiber laser.
FIG. 7 is a spot diagram for the collimation device shown in FIG. 6.
8 shows the invention configured as a focal objective for a high power fiber laser.
FIG. 9 is a spot diagram for the focal objective lens shown in FIG. 8. FIG.
FIG. 10 is an enlarged view of ray tracing of the objective lens shown in FIG. 8.
FIG. 11 is a spot diagram of fused silica and CaF2 doublets thermally optimized for minimum focal shift shift
12 is a cross-sectional view of the lens fixing device.
FIG. 13 is a cross-sectional view taken along lines 13-13 in FIG. 12.
14 is a cross-sectional view taken along lines 14-14 in FIG. 12.

1 is a schematic representation of a conventional lens arrangement, the whole of which is represented by reference numeral 10.

In particular, FIG. 1 shows a high power fiber laser emitted from the output of fiber 12. Lens 14 collimates light 16, lens 18 focuses the light back to focus 20, which focus 20 can be cut, drilled, scribed, marked, welded or processed. Is formed on the surface of the material. Both lenses are made of high quality fused silica.

FIG. 2 shows the lenses of FIG. 1 but a multi-row configuration is set up, where lenses 14 and 18 have an ambient temperature of 25 ° C., respectively, and then increase to 50 ° C., 75 ° C. and 125 ° C., respectively. Focus 20 shifts towards the lens due to an increase in temperature. Each configuration is overlaid with an offset of 2 millimeters (2 mm) to show a change in focus as the temperature is increased.

3 is an enlarged view of the focus of FIG. 2. The focus shift from the ambient environment at the top of FIG. 3 to the highest temperature at the bottom of FIG. 3 is 0.644 mm.

4 shows a conventional fiber collimator for a high power fiber laser 22. The fused silica doublet 24 comprising the planar convex lens 24a and the double-sided convex lens 24b produces an appropriate collimation level with minimum wavefront error.

FIG. 5 is a spot diagram of the collimating lens doublet 24 shown in FIG. 4. The airy disc radius (diffraction limit divergence) for this lens system is 0.074 mradian. As the temperature values increase from the previous 50 ° C, the rays begin to diverge beyond the diffraction limit, and the outermost sets of rays extend to 0.161 mradian at 125 ° C.

6 shows that the present invention is configured as a fiber collimator for a high power fiber laser. The first lens 26 is a fused silica lens, and the second lens 28 is made of Schott N-PSK53A.

FIG. 7 is a spot diagram for the novel collimation device shown in FIG. 6. At higher temperatures the divergence of the system is reduced to 0.083 mradian, which is very close to the diffraction limit of 0.0701 mradian.

8 shows the invention as a focal objective for a high power fiber laser. The first lens 30 is a fused silica lens, and the second lens 32 is made of Schott N-PSK53A.

FIG. 9 is a spot diagram for the focal objective lens shown in FIG. 8. FIG. The rms spot radius is 2.129 microns and the geometric radius is 3.936 microns, which is below the diffraction limit of 8.737 microns.

FIG. 10 is an enlarged view of the ray tracing of the objective lens shown in FIG. 8, when the temperature of the lens system increases over a range of 25 ° C., 50 ° C., 75 ° C. and 125 ° C. from top to bottom, respectively, FIG. ) Shows no discernible difference.

FIG. 11 is a spot diagram of fused silica and CaF2 doublets thermally optimized for minimum focus shift variations. FIG.

12 is a cross-sectional view of the lens fixing device 40. The device 40 includes an annular compression end cap 42 that cooperates with the annular reference end cap 44 to secure the annular main housing 46 therebetween, as shown in FIG. 13. The lenses 30, 32 are separated at their respective peripheral edges by an annular invar separator 48 radially located inside the main housing 46 as shown. The annular retaining ring 50 cooperates with a plurality of circumferentially spaced wave spring sheets, denoted 52, to fix the annular wave spring 54 therebetween. Wave spring 54 is about 65 pounds (65 lbs).

The lenses are cooled by separate circuits in fluid communication with the four couplings 56 as shown in FIG. 13. The intra-lens gas appliance 58 shown in FIGS. 12 and 14 delivers fluid from a remote gas source, preferably helium, having a thermal conductivity of 0.142 W / m ° C., which is nearly six times greater than air (0.024 W / m ° C.). It is thus possible to better conduct heat away from the lenses into the space between the lenses 30, 32. Port 60 provides a vent for the gas. If helium is used, helium may be recycled, and if air is used, the air may be transported away from the lens assembly. In any case, the cooling gas should be free of residues or particles that may contaminate the lens.

Couplings 56 and 58 are water cooled input and output ports. Each lens has its own cooling source, and water must be circulated to help adjust the temperature of the lens assembly. Tens of kilowatts of lasers generate enormous amounts of heat on the assembly from both absorbed and back reflected light at the lens surface. For example, if a loss of 0.5 percent (0.5%) per surface occurs for two element optics, this represents a 2 percent (2%) loss, and 2 percent (2%) of 10 kilowatts (10 ms) 200 watts (200W). That is, a significant amount of power is wasted.

In all embodiments, at least one of the elements forming the multi-element lens has a negative dn / dT value and at least one other lens has a positive dn / dT value.

Fused silica elements and N-PSK53A elements are employed in preferred embodiments. N-PSK53A glass is a moldable glass. Formability is advantageous for fabricating aspherical surfaces that help to reduce spherical aberrations. N-PSK53A can limit the laser power level used, as it has a softening point more than twice that of fused silica.

The second embodiment includes a CaF 2 element and a fused silica element. Both CaF 2 and fused silica are good materials for high power lasers ranging from UV to near infrared. The present invention is not limited to these two materials. The following table shows useful glasses with negative dn / dT values and corresponding α values:

Glasses typically used for high power lasers and their corresponding dn / dT and α values include the following:

The tables show that a perfect match of CTEs comparable to dn / dT cannot be obtained. The thickness and corresponding curvature of the lens elements are variables that can be optimized for optimal non-thermal activation of high power laser optics. This is accomplished by forming the appropriate merit function in the optical design software program to optimize the spacing, lens thickness and curvature to maintain the focus shift at nominal Rayleigh distance values for the lens system. The tables configure the combination of glasses to achieve non-thermal activation for high power lasers ranging from UV to far infrared, including but not limited to excimer, Nd: YAG, Nd: YLF, fiber and CO ₂ lasers. Indicates what can be done.

The table below shows the optical measures and thermal settings for a fused silica lens with a nominal effective focal length of 100 mm, focusing light at 1.075 microns, which is a typical wavelength of high power fiber lasers. The postfocal distance is provided for each temperature change, and the corresponding difference in focus shift to nominal temperature is provided as "delta" values. Positive "deltas" indicate a decrease in focus towards the lens and negative "deltas" indicate an increase in focus away from the lens.

The following table shows the optical measures for fused silica doublet at 1.075 microns.

The following table provides the preferred embodiment of action for a doublet focusing a fiber laser at 1.075 microns.

The focus shift over the 25 ° C. to 125 ° C. temperature range is only about 3 to 4 microns, which is 47 times improvement over the fused silica doublet in the table above.

The N-PSK53A glass cannot accommodate extremely high laser power. Thus, the table below shows the measures by which fused silica is used with CaF ₂ .

The presence of CaF ₂ causes the focal length of the system to increase by 53 microns as indicated by a negative sign in contrast to the fused silica doublet. The focus shift is 3.5 times improved over the prior art, and optics can be used with very high average power levels. The spot diagram of FIG. 11 shows that the size of the spot decreases as the lens system is heated. Thus, even when the focal length slightly increases, aberration is further reduced, the performance of the lens system is improved, and therefore diffraction is further limited.

To further enhance the performance of the optical design, the separation ring is made of a low thermal expansion material such as Invar with a CTE of 1.5 × 10 ⁻⁶ / ° C. This ring is a split ring over the outer diameter of each element, and the split allows for the expansion of the glass. The invar separating ring adds means for conducting heat away from the lenses. Both sides of the Invar separator are further machined to follow the contours of the lenses for maximum area contact on the first few millimeters and their circumference radially across the lenses. The main housing of the objective lens assembly is made of brass material with water-cooled ports and virtually no liquid contacts the lens. The Invar separator has a gas exhaust hole 180 degrees opposite the split, so clean, dry purge gas can be used to further cool the optical elements. Optics are added to the final surface of the assembly and fixed by a wave spring facing the first element. The assembly is allowed to extend toward the laser rather than the product, with the additional focus shift being further reduced due to the thermal expansion of the materials involved.

Accordingly, the objects described above and the objects recognized from the foregoing description are effectively attained, and any changes in the foregoing arrangements may be made without departing from the scope of the present invention and are included in the foregoing description and illustrated in the accompanying drawings. All matters to be addressed will be construed as illustrative, and not as restrictive.

It is to be understood that the following claims are intended to cover all descriptions of the scope of the invention as shown between the general and specific features of the invention described herein and between the claims as language problems.

Claims (7)

A method of thermally compensating lenses in an optical system for high power lasers,
Providing a positive dn / dT lens for collimating a high power laser beam, and positioning the positive dn / dT lens in a collimating relationship with the high power laser beam;
Providing a focus lens assembly for focusing the high power laser beam, and positioning the focus lens assembly in a focusing relationship with the collimated laser beam; And
Including at least one lens having a negative dn / dT as part of the focal lens assembly to counteract a thermally induced change in refractive index of the positive dn / dT lens. How to compensate. The method of claim 1,
Selecting from the group of lenses with negative dn / dT at least one lens having a negative dn / dT substantially equal to the positive dn / dT of the positive dn / dT lens,
Thus, the positive dn / dT lens in the optical system is balanced with the negative dn / dT lens that cancels out so that the optical system maintains focus over a wide temperature range. The method of claim 1,
Providing a positive dn / dT lens in the form of a fused silica lens in a focal lens assembly for focusing the collimated laser beam on one surface and providing at least one lens with the negative dn / dT N-PSK53A lens It further comprises providing in the form of,
Accordingly, the N-PSK53A lens compensates for the thermal power of the fused silica lens such that the total power of the optical system is not substantially changed. A method of thermally compensating lenses in a collimating optical system for high power lasers,
Forming the collimating optical system by positioning the fused silica lens and the N-PSK53A lens in a cooperating relationship with each other and a collimating relationship with a beam of light emitted by a high power laser. A method of thermally compensating lenses in a focal objective lens system for high power lasers,
Forming the focal objective lens system by positioning the fused silica lens and the N-PSK53A lens in a cooperating relationship with each other and with a beam of light emitted by a high power laser to form the focal objective lens system. A method of thermally compensating lenses in a focal objective lens system for high power lasers,
The focal objective lens system is formed by positioning a fused silica lens and a lens selected from a group of lenses formed of CaF2, BaF2, LiF2, NaCl and KCl glasses in a cooperating relationship and a focal objective with a beam of light emitted by a high power laser. And thermally compensating for the lenses. A method of thermally compensating lenses in a focal objective lens system that passes a collimated beam of light emitted by high power lasers,
Forming the focal objective lens system by positioning a fused silica lens and a lens selected from the group of lenses formed of CaF 2, BaF 2, LiF 2, NaCl and KCl glasses in a cooperating relationship and a focusing relationship with the collimated beam of light. How to thermally compensate the lenses.

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